High resolution projection systems are used to project an image of a mask onto the surface of a semiconductor wafer coated with a chemical photoresist during the fabrication of semiconductor circuits. Exposed regions of photoresist within the image of the mask are chemically altered and react differently to subsequent chemical or physical treatment of the wafer than unexposed regions. A series of masks and intervening treatments are used to form layers on the wafer having the required electronic structures.
The masks used in the process are expensive and time consuming to produce. Further changes required in the circuitry after the mask is produced typically require a new mask to be created. The complex optical systems used in the process are also expensive and require significant maintenance. High numerical aperture lenses have small depths of field and are limited in contrast at higher spatial frequencies. The demanding requirements of the semiconductor industry for higher resolution, contrast, depth of field and optical efficiency are coupled with a desire to minimize distortion. Physical constraints inherent in projection lens systems will limit further performance improvements.
The present invention overcomes the problems associated with the use of physical masks and lenses.